Substituent Effects In Parallel- Displaced Pi-Pi Stacking Interactions: Distance Matters

Host-guest systems with Rebek imide type receptors and a 2,6-di(isobutyramido)pyridine ligand were employed to investigate substituent effects in parallel-displaced pi-pi stacking interactions. Changing the intermolecular distance between the para substituent on the aromatic platform of the receptor and the pyridine ring of the guest results in a strongly different substituent effect. With a short ethyne-1,2-diyl spacer between the Rebek imide and the aromatic platform, partial overlap of substituent and guest stabilizes the pi-pi stacking interactions independent of the electronic nature of the substituent (Wheeler-Houk model). When the substituent is shifted further away by using a buta-1,3-diyne-1,4-diyl spacer, direct, through-space interactions between substituent and guest are prevented. A linear correlation between log K-a (K-a=association constant) and the Hammett substituent constant spara is observed, confirming predictions by the Hunter-Sanders model experimentally.